Civil Engineering Reference
In-Depth Information
To destratify the reservoir, the cooler, more dense water from the hypolimnion is
raised to mix with the less dense, warmer water of the epilimnion. If sufficient energy
is added, complete dispersion of the thermocline occurs. If the oxygen resources and
natural reservoir reaeration are sufficient, an aerobic reservoir may be maintained.
There are potential problems associated with destratification. In an advanced eu-
trophic reservoir, the accumulation of sufficiently high organic content in the hypolim-
nion may, if mixed with the epilimnion, cause insufficient dissolved oxygen throughout
the reservoir. In such a case, thermal destratification could do more harm than good.
In eutrophic reservoirs, dissolved nutrients in the hypolimnion, if mixed into the
epilimnion, may stimulate aquatic growths, which can trigger taste and odor problems
in the water supply. Also, mixing of water from the cool hypolimnion water with that
of the warm epilimnion will result in an increase in water temperature at a hypolimnitic
intake. The warmer water may not be desirable for a water supply.
Most of these problems appear to be of consequence only in small lakes. The studies
done with thermal destratification in large reservoirs have not shown evidence of either
oxygen depletion or increased phosphorus concentration in the epilimnion. Methods
to aerate the hypolimnion without mixing it with epilimnion have been developed by
several investigations.
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The most common method is to install a tube the full depth
of the reservoir with the upper lip of the tube above the reservoir water surface. Air
is injected near the bottom of the tube, and an airlift effect is produced, raising the
water in the tube and concurrently aerating the water in transit. A slot or opening is
provided in the tube below the thermocline to provide a means for the water to escape
before it reaches the epilimnion, and the air continues up the tube to the atmosphere.
The two methods for in-reservoir oxygenation are:
Thermal destratification to co-mix the epilimnion and hypolimnion to provide a
uniform dissolved oxygen concentration and temperature,
•
Artificial oxygenation and mixing of the hypolimnion to maintain a layered res-
ervoir.
•
The energy levels required to accomplish thermal destratification have been cal-
culated by Symons, et al., who related energy efficiency of destratification to ''oxy-
genation capacity'' (OC) and ''destratification efficiency'' (DE).
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The oxygenation capacity is determined by calculating the mass input of dis-
solved oxygen per unit of time divided by the energy input per unit of time. The
dissolved oxygen input per unit of time is the summation of increased concentration
in dissolved oxygen per unit of time and the steady-state demand rate for oxygen per
unit of time. In reservoirs, the demand rate for oxygen is low and often ignored as
insignificant. The OC is expressed in units of pounds of oxygen transferred per kilo-
watt-hour.
The DE is calculated by the net change of ''stability'' over a set time frame divided
by the total energy input over the same time frame. Stability is defined as the minimum
energy needed to mix the lake, and is calculated by multiplying the weight of water
in the lake by the vertical distance between the center of gravity of the lake, taking
into account the density due to the thermal gradient. The results of this calculation
are in foot-pounds, which can be converted to kilowatt-hours over the time frame used.
The change in stability divided by the power input yields the DE.
As the depth of the epilimnion increases and / or as the temperature differential
between the epilimnion and hypolimnion increases, the location of the center of gravity
